Modern internal combustion engines cannot usually be started from rest without an auxiliary unit. In the cylinder or cylinders of internal combustion engines, in which the fuel is fed into the combustion air within the induction section, there is, as a rule, no ignitable air/fuel mixture, or only a little, so that activating the ignition source of a spark ignition unit is not sufficient to start combustion. Therefore, an auxiliary drive (usually electrically operated), referred to as a starter, is used, to bring the internal combustion engine to a starting speed. At this speed, sufficient ignitable mixture is sucked into the cylinders, ignited by means of a spark ignition device and the internal combustion engine is thus started. The starter can then be switched off and the internal combustion engine continues to rotate under its own power.
The situation is different in the case of internal combustion engines in which the fuel is introduced directly into at least one cylinder, the fuel usually being injected under pressure into the cylinder. If fuel is injected into one or more cylinders and subsequently ignited inside a stationary internal combustion engine, the resulting gas force can be sufficient to move the associated piston. The movement of the piston imparts angular momentum to the crankshaft via a connecting rod, and the angular momentum can be sufficient to induce the normal sequence of cycles of an internal combustion engine. In this way, the internal combustion engine can be started up or started directly without using a starter.
A plurality of preconditions have to be fulfilled to allow an internal combustion engine to start directly in the above-described manner. For instance, at least one piston of a cylinder of the internal combustion engine should be in a position, in which injecting fuel and subsequently igniting the created air/fuel mixture actually leads to a movement of the piston located in the cylinder. In the case of a four-stroke internal combustion engine, the piston would therefore have to be in the expansion or working cycle, and the at least one outlet gas exchange valve would have to be in a closed position. This is the case, for example, at a position of approximately 5° to 10° crank angle after top dead center (TDC). The piston would also move in the case of a position of approximately 5° to 10° before TDC. However, the crankshaft would then move counter to the desired direction of rotation. U.S. Pat. No. 6,098,585 describes, by way of example, a method for direct starting. Here, a suitable combustion chamber is identified, which is formed from a cylinder and piston, which is in the working cycle and contains a sufficient volume of air. A predetermined amount of fuel is subsequently injected and ignited. However, U.S. Pat. No. 6,098,585 assumes that a starter, even if of relatively small dimensions, is nevertheless required for starting the internal combustion engine.
A somewhat different approach is chosen in U.S. Pat. No. 6,453,863. Here, the crankshaft is moved into a preferred position by a starter motor which is mounted directly on the crankshaft and serves after starting as a generator, what is referred to as a starter generator. In said preferred position, it is possible to make it considerably easier, for example, to start the internal combustion engine using the above-described method of injecting fuel and subsequently igniting it.
In WO 01/48373, the desired piston position is achieved by the piston already being moved to the desired position when the internal combustion engine is switched off. As the internal combustion engine continues to move for a few revolutions after the ignition and the fuel supply have been switched off, on account of the engine's moment of inertia, until it has come to a standstill as a result of frictional and gas forces, it is possible for a suitable position for direct starting to be reached, for example, by switching the engine off in a controlled manner, by advancing the crankshaft by means of an auxiliary drive, or by executing additional individual working cycles during the switching off process.
The abovementioned methods have the disadvantage, however, that they only concentrate on reaching a favorable piston position to perform direct starting by means of auxiliary drives or to make the process of starting an internal combustion engine easier. In the process, they neglect the fact that relatively large amounts of products of combustion may still be located in the cylinder or cylinders, so that it is not possible to carry out direct starting, which may possibly take place later, the above-described method on account of a lack of oxygen in the cylinder or cylinders.